(i) Field of the Invention
The present invention relates to a tap device of a cable broadcasting system in which a part of transmission signal running through a transmission line leading to a terminal side from a center apparatus is branched and transmitted to a subscriber""s terminal device, particularly to a tap device of a cable broadcasting system in which it can be switched on the side of the center apparatus whether or not the transmission signal is transmitted to the terminal device.
(ii) Description of the Related Art
In a conventional cable broadcasting system such as CATV system in which a broadcasting signal such as a television signal is transmitted to a subscriber""s terminal device via one transmission line formed of a coaxial cable, and the like, the transmission line is provided with a tap device, so-called tap-off, for leading signals in order to branch the broadcasting signal from the transmission line and leading the signal into a subscriber""s house.
Moreover, in a known tap device, it can easily be switched according to a subscriber""s request or the like whether or not the broadcasting signal is distributed to the subscriber""s house. Specifically, the tap device is provided with a relay disposed in a signal path between a directional coupler for branching a part of the broadcasting signal from the transmission line and a branched output terminal for outputting the branched broadcasting signal toward the subscriber""s terminal device, in which a high-frequency broadcasting signal can be passed with a reduced loss. The relay is a so-called high-frequency relay, and a latching relay which can hold a switched state is usually used. By driving the relay in response to a command signal transmitted from the center apparatus via the transmission line, the connected/disconnected state of the signal path, in other words, the output or cut-off of the broadcasting signal to the terminal device can easily be switched on the side of the center apparatus.
When the conventional tap device is installed on the transmission line, an individual address is allocated to the device in order to distinguish the device from the other electronic apparatuses connected to the transmission line. Additionally, when the center apparatus switches the output of the broadcasting signal to the branched output terminal of the tap device, a command signal including command data and the individual address of the tap device is generated. This command signal is sent to the transmission line. In this case, on the side of the tap device, the command signal having the corresponding address is selected from various command signals transmitted via the transmission line, that is, the command signal from the center apparatus is extracted. By driving the relay based on a command included in the command signal, it is switched whether or not to transmit the broadcasting signal to the branched output terminal.
Therefore, according to the conventional tap device, it can easily be switched on the side of the center apparatus whether or not to transmit the broadcasting signal to the subscriber""s terminal device from the branched output terminal. Therefore, an operator does not have to go to a place where the tap device is installed for the switching operation.
Additionally, in the conventional tap device, in order to receive the command signal from the center apparatus to drive the relay, electric power has to be supplied from the outside for the switching operation. Furthermore, it is troublesome to place a separate cable for the power supply, which enormously increases costs.
In the cable broadcasting systems such as CATV system, the power is usually supplied to the electronic apparatuses such as an amplifier connected to the transmission line via the transmission line. Basically, the broadcasting signals or high-frequency signals such as television signals are transmitted via the transmission line. However, by using a frequency band different from that of the broadcasting signal, the above-described command signal and the power signal can simultaneously be transmitted. Therefore, in the conventional system, by supplying an alternating-current power signal of several tens of Hz(hertz) to the transmission line with several tens of V(volts), the alternating-current power signal is used to supply electric power to various electronic apparatuses connected to the transmission line.
For this purpose, the tap device is usually provided with a power supply circuit in which the alternating-current power signal supplied to the transmission line is taken into the device, and rectified and smoothed to generate a direct-current voltage for operating an inner circuit. With the direct-current voltage generated by the power supply circuit, a drive circuit for driving the relay, a receiving circuit for receiving the command signal, a control circuit for turning on/off the relay via the drive circuit based on the received command signal, and the like are operated.
In the receiving circuit and the control circuit of the conventional tap device, semiconductor integrated circuits can be used, and the operation voltage can be set to a low voltage of 5V or 3.3V as in a general one-chip microcomputer. However, a high voltage is advantageous for the relay switch, because the relay can be driven with less current.
Therefore, the power supply circuit to be incorporated into the tap device is preferably constituted such that a direct-current high voltage of 10V or more (hereinafter referred to as the first power voltage) is supplied to the relay drive circuit, and that a direct-current low voltage lower than the first power voltage (hereinafter referred to as the second power voltage) can be supplied to the receiving circuit and the control circuit.
In the conventional tap device, for example, the power supply circuit is constituted so as to rectify/smooth the alternating-current power signal transmitted via the transmission line, then generate the first power voltage using a three-terminal regulator, a Zener diode, and the like, and further to generate the second power voltage for driving the control circuit from the first power voltage.
On the other hand, since each cable broadcasting system in each service area is independent, the alternating-current power signal supplied to the transmission line differs in each system, and the voltage of the alternating-current power signal differs by about several tens of V(volts) among systems. Therefore, when the tap device is used only in a specific system in which the alternating-current power is constant, the power supply circuit to be incorporated into the tap device may be designed in accordance with the alternating-current power to be used so that the power loss is minimized. However, when the tap device is provided with general-purpose properties so that the tap device of the same specification can be used in the systems different from one another in terms of the voltage of the alternating-current power, a problem arises that the power loss in the power supply circuit is increased in the system with a high voltage.
Specifically, for example, when the common tap device is used in the systems in which the voltages of the alternating-current power signals are 45V and 90V, the power supply circuit needs to generate the first and second power voltages in the range of the alternating-current power signal of 45V to 90V. In this case, the power supply circuit has to be designed such that even when the voltage of the alternating-current power signal is 45V, the first and second power voltages can be generated and that when the voltage of the alternating-current power signal is 90V, the power for the voltage difference is consumed by the three-terminal regulator, the Zener diode, and other components forming the power supply circuit. Therefore, when the conventional tap device can be utilized in the systems different from one another in terms of the voltage of the alternating-current power signal running through the transmission line, the power loss in the power supply circuit is increased in the system in which the voltage of the alternating-current power signal is high.
When the power loss in the power supply circuit is increased as described above, another problem arises that the thermal emission from the power supply circuit is increased to adversely affect the operations of the receiving circuit and the control circuit, or that the power consumption of the entire system is increased and the power capacity of the power supply circuit for supplying the alternating-current power signal to the transmission line has to be enlarged.
Although, it can be considered to solve these problems that a switching regulator excellent in conversion efficiency is used in the power supply circuit, such a switching regulator has a complicated constitution and is expensive. Additionally, since switching noises are generated during the voltage converting operation, the switching regulator cannot be employed in the tap device of the cable broadcasting system. Specifically, in the tap device, the high-frequency broadcasting signals are received and transmitted. Therefore, if the switching regulator is incorporated, the switching noises are superimposed to the broadcasting signals transmitted to the transmission line and the subscriber""s terminal device from the tap device, thereby deteriorating the broadcasting signals. Therefore, the switching regulator cannot be incorporated into the tap device.
Wherefore, an object of the present invention is to provide a tap device for a cable broadcasting system, which can switch between outputting and stopping a broadcasting signal via a branched output terminal in response to a command signal transmitted from a center apparatus via a transmission line for transmitting the broadcasting signal, which receives an electric power for the switching operation via the transmission line, which can be used in systems different in voltage of power signal, and which reduces power loss in a power supply circuit generated by the difference in voltage.
To attain this and other objects, according to one aspect of the present invention, in the same manner as in the conventional tap device, there is provided a tap device which is connected to a transmission line for transmitting a broadcasting signal to a terminal side from a center apparatus, and which is provided with a directional coupler for branching a part of a transmission signal including the broadcasting signal on the transmission line and transmitting the branched transmission signal to terminal devices via branched output terminals. Moreover, a signal path for leading the branched transmission signal to the branched output terminal from the directional coupler is provided with a latching relay for switching the connected/disconnected state of the path, and the latching relay can be driven or switched on/off via a drive circuit. Additionally, the latching relay is a known relay also called a keep relay or a holding relay. By energizing a relay coil, the position of a movable contact can be switched. After the switching, even if the electricity to the relay coil is cut off, the position of the movable contact can be self-retained.
On the other hand, in the tap device of the present invention, a receiving circuit receives a command signal transmitted from the center apparatus via the transmission line, a control circuit extracts the command signal for the tap device from the command signals received by the receiving circuit, a control signal is transmitted to a drive circuit based on the extracted command signal, and the connected/disconnected state of the signal path by the latching relay is switched. Therefore, when the broadcasting signal is distributed to a subscriber""s house via the tap device connected to the transmission line of the cable broadcasting system, and the subscriber requests to temporarily stop off the distribution of the broadcasting signal, the output of the broadcasting signal to the subscriber""s terminal device can be stopped by sending the command signal for turning off the latching relay of the tap device to the transmission line from the center apparatus.
In the tap device constituted as described above, in order to operate the receiving circuit, the control circuit, and the drive circuit, each circuit requires a power supply circuit for supplying the direct-current voltage. The tap device of the present invention is provided with two power supply circuits: a first power supply circuit for supplying a first power voltage to the drive circuit to drive the relay; and a second power circuit for supplying a second power voltage lower than the first power voltage to the receiving circuit and the control circuit to operate these circuits.
In the same manner as the power supply circuit of a conventional tap device, the power supply circuits generate the power voltages from the alternating-current power signal supplied to the transmission line in the present invention. However, the first power supply circuit rectifies/smoothes the alternating-current power signal supplied to the transmission line, and generates the first power voltage necessary for the drive circuit to drive the latching relay from the rectified/smoothed power signal. The second power supply circuit once lowers the voltage of the alternating-current power signal in a transformer, rectifies/smoothes the alternating-current power signal with the lowered voltage, and generates the second power voltage necessary for operating the receiving circuit and the drive circuit from the rectified/smoothed power signal.
Therefore, when the tap device of the present invention is used in the cable broadcasting systems with different alternating-current power signals employed, the power loss generated in the system in which the voltage of the alternating-current power signal is high is prevented from becoming remarkably large by the voltage change of the alternating-current power signal. Different from the conventional tap device, the power loss in the power supply circuit can be reduced.
Specifically, in the second power supply circuit of the present invention, after the voltage of the alternating-current power signal supplied to the transmission line is once lowered by using the transformer, the signal is rectified/smoothed, and the second power voltage is generated from the rectified/smoothed power signal. Therefore, the voltage change of the alternating-current power signal transmitted to the circuit portion or constant-voltage circuit for generating the second power voltage can be smaller as compared with when no transformer is used. As a result, the power loss generated in generating the second power voltage from the rectified/smoothed power signal can be reduced.
On the other hand, in the present invention, the first power supply circuit for generating the first power voltage to drive the relay is provided with no transformer, and the alternating-current power signal supplied to the transmission line is directly rectified/smoothed for the following reasons.
First, the first power voltage generated by the first power supply circuit is a power voltage for driving the latching relay to switch on/off the relay. Therefore, as the voltage is increased, the current passing through the relay coil at the time of driving the relay can be reduced, and the power consumption required for the switching of the latching relay can be reduced. Therefore, the first power voltage is preferably set to be as high as possible relative to the second power voltage, and it is preferable to directly rectify/smooth the alternating-current power signal without lowering the voltage of the alternating-current power signal in the transformer as in the second power supply circuit.
Moreover, when the voltage of the alternating-current power signal is not lowered with the transformer, the voltage change of the power signal transmitted to the constant-voltage circuit in which the first power voltage is generated from the rectified/smoothed power signal becomes larger than that in the second power supply circuit, and the power loss generated in the constant-voltage circuit is increased in the system in which the voltage of the alternating-current power signal is high. However, the latching relay is driven only when the command signal is transmitted to the tap device from the center apparatus and the ON/OFF state needs to be switched. Therefore, the power loss in the first power supply circuit is only temporarily generated when the latching relay is driven, and does not cause either thermal emission or the power consumption of the entire system to be increased.
As described above, according to the present invention, even if the range of allowable voltage of the alternating-current power signal to each power supply circuit is set wide in order to utilize the tap device in the systems with different voltages of the alternating-current power signal supplied to the transmission line employed, the thermal emission due to the power loss in the power supply circuit does not become a problem in the system in which the voltage of the alternating-current power signal is high. The power consumption of the tap device can be reduced.
Moreover, when the latching relay is driven, the relay coil can be energized with the first power voltage which is higher than the second power voltage, and the amount of the current running through the relay coil can be reduced. Therefore, the power consumption required for switching the latching relay can also be reduced.
Furthermore, since the transformer is used in the second power supply circuit, it can prevent surge voltages or noise signals generated by lightning and other disturbances from entering the second power supply circuit. Therefore, according to the present invention, in the second power supply circuit, a stable second power voltage can always be generated without being influenced by the disturbances, so that the receiving circuit and the control circuit can stably be operated.
Here, in each power supply circuit, since the first power voltage or the second power voltage is generated from the rectified/smoothed power signal, the constant-voltage circuit is incorporated. According to the present invention, the constant-voltage circuit of the first power supply circuit is constituted to control the first power voltage corresponding to the breakdown voltage of the Zener diode by applying a reverse bias to the Zener diode with the rectified/smoothed power signal, and the constant-voltage circuit of the second power supply circuit is constituted of a three-terminal regulator.
Specifically, the first power voltage is generated for the drive circuit to drive the latching relay. Even if the voltage level slightly fluctuates, the latching relay does not malfunction. Additionally, since the latching relay is temporarily driven, the power consumption of the tap device is not increased by the operation of the first power supply circuit. Therefore, in the tap device of the present invention, by constituting the constant-voltage circuit incorporated in the first power supply circuit with the constant-voltage circuit of the Zener diode, the first power supply circuit can inexpensively be realized.
On the other hand, the second power voltage is generated so as to operate the receiving circuit and the control circuit. If the voltage fluctuates, the circuits can malfunction in some cases. To solve the problem, in the tap device of the present invention, by using the three-terminal regulator which can generate a stable constant voltage as the constant-voltage circuit of the second power supply circuit, the second power voltage is stabilized.
According to the tap device of the present invention, in addition to the above-described effects, there can be provided effects that the second power voltage is stabilized more securely and the receiving circuit and the control circuit can be operated constantly normally, and that the first power supply circuit is constituted inexpensively and thus the cost of the tap device can be reduced.
On the other hand, the tap device of the present invention is provided with a spliting circuit for distributing the transmission signal branched by the directional coupler into a plurality of signals, and transmitting the distributed transmission signals via a plurality of branched output terminals. In this case, a plurality of latching relays are disposed in a plurality of signal paths connected to a plurality of branched output terminals from the splitting circuit. The control circuit controls the individual connected/disconnected states of the broadcasting signal paths by the latching relays via the drive circuit based on the command signal transmitted to the tap device from the center apparatus.
Specifically, according to the tap device of the present invention, the broadcasting signal can be distributed to a plurality of subscribers"" terminal devices via the transmission line. Furthermore, since the signal paths leading to a plurality of branched output terminals from the splitting circuit are provided with the latching relays, by individually switching the ON/OFF states of the latching relays, it can be set whether or not to transmit the broadcasting signal to each subscriber connected to each branched output terminal. Therefore, according to the tap device of the present invention, the broadcasting signal can be distributed to a plurality of subscribers"" terminal devices, and additionally the center apparatus side can switch the distributing/stopping of the broadcasting signal to each subscriber.
Also in this case, the first power voltage for driving the relay is generated in the first power supply circuit, and the second power voltage for operating the receiving circuit and the control circuit is generated in the second power supply circuit. Therefore, in the same manner as described above, the effects can be obtained that the power loss of the power supply circuit generated in the system with the high voltage of the alternating-current power signal can be reduced and the power consumption in the tap device can be suppressed.